An open lake is a lake where water constantly flows out under almost all climatic circumstances. Because water does not remain in an open lake for any length of time, open lakes are usually fresh water: dissolved solids do not accumulate. Open lakes form in areas where precipitation is greater than evaporation. Because most of the world's water is found in areas of highly effective rainfall, most lakes are open lakes whose water eventually reaches the sea. For instance, the Great Lakes' water flows into the St. Lawrence River and eventually the Atlantic Ocean.
In a closed lake (see endorheic drainage), no water flows out, and water which is not evaporated will remain in a closed lake indefinitely. This means that closed lakes are usually saline, though this salinity varies greatly from around three parts per thousand for most of the Caspian Sea to as much as 400 parts per thousand for the Dead Sea. Only the less salty closed lakes are able to sustain life, and it is completely different from that in rivers or freshwater open lakes. Closed lakes typically form in areas where evaporation is greater than rainfall, although most closed lakes actually obtain their water from a region with much higher precipitation than the area around the lake itself, which is often a depression of some sort.
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Igneous rocks are defined as formations made from solidified lava or magma.
When lava or hot Magma reaches the surface of the earth or a water body it quickly cools down and changes it's from from a hot liquid to a solid rock.
The chemical composition of Igenous rocks can vary depending on their location, cooling time etc.
However, most Igenous rocks contain some or all of the following minerals:
1. Quartz
2. Micas
3. Amphiboles
4. Olivines
5. Pyroxenes
6. Feldspathoids
7. Feldspars
They can be replaced naturally.
